Chronic myelogenous leukemia (CML) and Ph-positive acute lymphoblastic leukemia (ALL) are characterized by a specific chromosomal translocation, the t(9;22) leading to the formation of the Philadelphia (Ph) chromosome. The ABL proto-oncogene from chromosome 9 is juxtaposed to the BCR gene on chromosome 22 by this translocation. The chimeric BCR/ABL gene produces P210 or P190 fusion proteins (reviewed in 1-3, 29, 30). The ABL protein is a tyrosine-specific protein kinase of the non-receptor type, and its tyrosine kinase activity is deregulated in BCR/ABL as a consequence of the attachment of the BCR moiety (31, 32). Specifically, ABL encodes a P145 non-receptor protein tyrosine kinase (PTK), and contains several other functional domains, including an SH3, an SH2 and an F-actin binding domain (reviewed in 4). Likewise, the BCR protein is multifunctional: the domain encoded by BCR exon-1 has serine/threonine kinase activity, can dimerize and binds the ABL SH2 domain in a non-phosphotyrosine-dependent manner (1, 5); the central part of the molecule has homology to GTP exchange factors (6); and its carboxy terminus has GTPase-activating protein (GAP) activity towards small p21.sup.ras -like molecules, including p21.sup.rac (7). Both BCR and ABL are thus implicated in signal transduction. The BCR/ABL p210 contains the BCR exon 1 encoded domain, the exchange factor homology domain and the majority of the ABL protein, and the oncogenic effect of BCR/ABL must be related to the perturbation of the normal BCR and/or ABL signaling pathways.
Aspects of a signaling cascade involving the EGF-receptor, a receptor protein tyrosine kinase, have recently been elucidated. Upon ligand binding, the EGF-receptor is auto-phosphorylated on tyrosine, thereby providing a binding site for GRB2. GRB2, which consists of a single SH2 domain flanked by two SH3 domains, associates with tyrosine phosphorylated proteins through its SH2 domain and with the RAS guanidine-nucleotide exchange factor mSOS1 through its SH3 domains (8; and reviewed in 9). Thus recruited to the plasma membrane, mSOS1 can activate p21.sup.ras. Recently, it has been demonstrated that GRB2 also binds BCR/ABL via Y.sup.177, a tyrosine residue encoded by BCR exon 1 only phosphorylated in BCR/ABL expressing cells (10, 11).
Through the use of experimental animal models, it has been shown that the BCR/ABL protein is sufficient to cause leukemia (33-35). Currently, the mechanism by which this takes place is largely unknown. BCR/ABL has been found in association with an increasing number of other proteins. A protein called ph-P53 complexes with BCR/ABL in the CML cell line K562 (36-37). The BCR protein itself is phosphorylated by and found in complex with BCR/ABL in K562 and transfected COS cells (37-39). Tyrosine-phosphorylated rasGAP, its associated proteins p190 and p62 and the adaptor proteins GRB-2 and SHC are also co-immunoprecipitated with BCR/ABL (10, 11, 40, 41). GRB-2 is constitutively associated with the p21.sup.ras nucleotide exchange factor SOS via its SH3 domains, and BCR/ABL might therefore intervene with normal cellular signaling by upregulating p21.sup.ras activity (10, 43). Increased tyrosine phosphorylation of a hematopoietic cell lineage specific tyrosine kinase, p93.sup.c-fes, has also been reported (44).
Several other "adaptor proteins" have been identified to date (12). CRK was initially discovered as the oncogene v-crk (13). CRK consists of an amino terminal SH2 domain and two tandem SH3 domains. Deletion of the carboxy terminal SH3 domain leads to transformation which is accompanied by an increase in cellular phosphotyrosine (14, 15, 28). A gene, CRKL, encoding a protein with a 60% overall homology to CRK, was recently isolated (16, 28). The CRKL gene was fortuitously identified through its location centromeric to the BCR gene on human chromosome 22 (26). CRKL consists solely of an SH2 domain and two tandem SH3 domains in the absence of a catalytic domain (16).
Presently CML and ALL patients are treated chemotherapeutically with conventional therapeutics and radiation. Such treatment is plagued by well-known side-effects and is often of limited effect. No effective treatment for these leukemias is known. Thus, other compositions and methods for treating such cancers are being sought.
There remains a need in the art for effective therapeutic compositions and methods to treat leukemia or ameliorate its effect on a human patient.